Multivibrator



A 1944- A. A. MACDONALD ETAL 2,356,071

' MULTIVIBRATORY Filed July 11, 1942 WITNESSES: INVENTORS Qc/ohflLJo/mson and Anylg ANacdona/a ATTORNEY Patented Aug. 15, 1944MULTIVIBRATOR Angus A. Macdonald and John L. .lohnson,

Catonsville, Md., assignors to ,Westmghouse Electric & ManufacturingCompany,

East

Pittsburgh, Pa., a corporation of Pennsylvania Application July 11,1942, Serial No. 450,506

Claims. (01. 25036) This invention relates to oscillation generators,and more particularly to generators of the type known as relaxationoscillators or multivibrators.

A particular feature of this invention is that in a multivibrator typeof oscillator simple means are provided for controlling the frequency ofthe oscillations within a wide range.

Another feature of this invention is that the novel control for thefrequency will not change the amplitude of the oscillations within acomparatively wide frequency range.

A particular advantage of the invention is that only slightmodifications are necessary to the standard multivibrator circuit forobtaining a wide range of frequency adjustment.

Another advantage resulting from the circuit arrangement herein proposedis that the frequency of a multivibrator may be varied by means of asingle control without changing the symmetry of the wave form of theoscillations produced.

The invention in its broader aspect comprises a coupling circuit forcontrolling the frequency of a relaxation oscillator in which there aretwo vacuum tubes so interconnected that the output circuit of one tubeis coupled to the input circuit of the other tube and vice versa, theoutput circuit of the latter is coupled to the input circuit of theformer. The couplin circuit above referred to provides aninterconnection between identical circuits of the tubes.

Other objects and advantages will be apparent from the followingdescription of the invention, pointed out in particularity by theappended claims, and taken in connection with the accompanying drawing,in which:

Figure 1 shows a schematic circuit of a multivibrator with a couplingcircuit interconnecting the input circuit of the tubes; and

Fig. 2 shows a modification of the circuit of Fig. 1 with a couplingcircuit interconnecting the output circuit of the tubes.

Circuits of the relaxation oscillator type are widely used in the artfor producing oscillations in a simple manner without tuned reactancenetworks. A multivibrator circuit may be regarded simply as a two-stageresistance coupled amplifier in which the voltage developed by theoutput of the second tube is applied to the first tube. The oscillationsare produced because in each tube there will be 180 phase shift, so thatthe output of the second tube will supply to the first tube an inputvoltage that has exactly the right phase to sustain oscillations. Inorder to change the frequency of the oscillations of a multivibrator, it

was the practice heretofore to vary the value of the coupling condensersor the resistors in the grid circuit. Since there are two resistors andtwo condensers, it is necessary to vary simultaneously these elements ifit is desired to maintain the symmetry of the output wave form as thefrequency is changed. Controlling the frequency in this manner has manydisadvantages. First of all, two controls are needed which also must beganged together for simultaneous variation of the abovementionedcomponents. It i often difificult and expensive to obtain gang controlswhose parameters will vary equally with equal control shaft rotation. I

Another methodheretofore used for varying the frequency of amultivibrator was to vary the bias voltage for the grid electrodessimultaneously for both tubes. The range over which the frequency of amultivibrator may be varied by this method isvery limited.

The frequency control herein proposed will be best understood by firstreferring to the figures of the drawing. The simple multivibrator shownin Fig. 1 comprises vacuum tubes l and 2 having anodes 3 and 3, cathodes4 and 4', and control electrodes 5 and 5', respectively. The inputcircuit of tube l between the cathode 4 and the control electrode 5comprises the grid load resistance 1, and similarly the input-circuitfor tube 2, the grid load resistance 1'. The output circuit includes forthe tube l the anode load resistance 9 for tube 2, the anode loadresistance 9 connected to the high potential side of the anode supplyshown here by the battery ill. The negative terminal thereof returns toground completing the circuit to the cathodes which are also at groundpotential. The output circuit of tube I also includes the couplingcondenser II which is connected between anode 3 and control electrode 5'of the tube 2. Similarly, the output circuit of the latter includes thecoupling condenser l2 connected between anode 3' and control electrode 5of tube l. The oscillations produced may be taken oif for a utilizationcircuit between terminals 15 and I6, which connect to the anode 3through a coupling condenser l1 and to the cathode 4, re-

spectively. V

The circuit so far described is a conventional multivibrator in whichthe frequency is determined by the constants of the symmetricallyarranged components, namely, the resistors l and l, 9 and 9', andcoupling condensers l I and 12.

The frequency control in accordance with this invention comprises thevariable resistance l8 and fixed resistance l9 inseries therewithconnected between control electrodes 5 and 5'. In other words, theseresistors form a circuit which interconnects identical circuits of thetwo tubes, namely, in this case, the input circuits of tube I and tube2. The variable resistor I8 is the effective frequency control meanswhereas the fixed resistor 19 is .a residual resistance provided toprevent .cessationof oscillations when the variable resistance I8 is atthe point of minimum resistance.

In addition the multivibrator circuit of Fig. also includes a certainamount of degenerative feedback for the tubes in the form of cathoderesistors 6 and 6' in the cathode return of tubes 1 and 2, respectively.The degenerative feedback so obtained is particularly'useful to reducethe change of amplitude encountered with change of frequency beyond acertain frequency range. The amplitude of oscillations will be more uniform over an extended frequency range with a certain amount of decreasein the voltage output of the oscillator. The inverse feedback utilizedin this simple form may, of course, be applied to the circuit shown inFig. las well. Other forms of degenerative feed-back circuits can alsobe employed.

By proper choice of the circuit constants, it was found that by'yaryingthe value of the resistance 118 a wide variation' in the frequencyrangeof the oscillator could be obtained. In a practical embodiment, avariation in the frequencyrange of 120:lwas readily obtained withoutgreatly influencing the amplitude of the oscillations. Even greaterfrequency ranges may be covered if appreciable amplitude variation withfrequency is not objectionable.

v 7 Referring to Fig. 2, the circuit shown here is in allitsessentialdetails similar to the oneshown in Fig. '1. Identicalcircuit elements are designated with similar reference characters. Onedifference is that the frequency control circuit comprising the variableresistor l8 and the fixed resistor 10 is connected here between theanodes 3 and 3 insteadof the grids 5 and 5' as in Fig. 1. In otherwords, theconnection is between the output circuits of the tubes I andZ.The output frequency of the oscillator is derived from tube 2 in tha;tthe terminals l5 and I6 connect to the anode? and cathode 4',respectively. The result obtained by this connection of the frequencycontrol circuitis similar to the one shown in Fig. 1, ahd the'saniewidevariation in frequency can be obtained, 7

In the operation of these circuits, when the control resistor i8 isturned to the point of maximum resistance, the frequency of themultivibrator osciilations will be determined by the time constant ofresistor 1 and condenser l2 plus the time constant of the resistor 1 andthe condenser H When the control 8 is rotated towards its minimumresistance value, the frequency of the multivibratoroscillations willincrease. This is due to the fact that the reactance of the condensersII and [2 at the oscillation frequency must be appreciably less than thecombined resistance of resistors l8 and I9, in order that oscillationsshall be sustained. For this reason, the value of the resistance I 8should be high compared to the resistances 7 and 1'. v

i In a practical embodiment of the oscillator circuit, the followingconstants for the components'resulted in an oscillator output of 22volts peak-to-peak over a range from /2 to 200 cycles. Resistors"! and?were each-3 megohms; coupling.

condensers H and I Z'e'ach two microfarads; and

resistors 9 and 9' each 1500 ohms. The source of anode potential was 300volts. The tubes used were of the type GSN'IGT. The control circuitvalues were 3 megohms for the variable resistance l8 and 10,000 ohms forthe fixed resistor NJ. The amplitude of the output was practicallyconstant over /2 of the frequency range with .slight variation for theentire range above stated.

It is clearly seen from the above description 2. very simple expedientis provided for adjusting the frequency of a relaxation oscillator. Bythe addition of only two components, of which only the flexibility ofthe oscillator circuit for various uses, rather it adds to it by makingthe variation of the output frequency extremely simple.

We claim as our invention:

1,. A relaxation oscillator comprising a pair of electron dischargetubes, an input circuit and an output circuit for each of said tubes,circuit means for coupling the output circuit of the first tube with theinput circuit of the second tube and for coupling the output circuit ofthe second tube with the input circuit of the first tube, and means forcontrolling the frequency of the oscillations generated comprising avariable resistor coupling identical points on said circuitsrespectively of said tubes.

2. An oscillatory system of the multivibrator type comprising tWoelectron discharge tubes each having an anode, a cathode, and a controlelectrode, condensers cross connecting the anodes and control electrodesof said tubes, a resistor connected in each of the anode circuits ofsaid tubes, a resistor connected between the control electrode andcathode of each of said tubes, and a connection including a circuitelement'of variable direct current conductivity between the controlelectrodes of said tubes whereby the frequency of the generatedoscillations may be varied in accordance with the variation of theconductivity of said element.

3. An oscillatory system of the multivibrator type comprising twoelectron discharge tubes each having an anode, a cathode, and a controlelectrode, condensers cross-connecting the anodes and control electrodesof said tubes, a resistor connected in each of the anode circuits ofsaid tubes, a resistor connected between control electrodes and cathodeof each of said tubes, and a connection including a circuit element ofvariable direct current conductivity between the anode electrodes ofsaid tube whereby the frequency of the generated oscillations may bevaried in accordance with the variation of the conductivity of saidelement 4. An oscillatory system of the multivibrator type comprisingtwo electron discharge tubes each having an anode, a cathode, and acontrol electrode, condensers cross-connecting the anodes and controlelectrodes of said tubes, a resistor connected in each of the anodecircuits of said tubes, a resistor connected between the controlelectrode and cathode of each of said tubes, and a variable resistancein series with a fixed resistance between the control electrodes of saidtubes whereby the frequency of the generated trodes and cathode of eachof said tubes, and

a variable resistance in series with a fixed resistance between theanodes of said tubes whereby the frequency of the generated oscillationsmay be varied in accordance with the variation of said variableresistance.

ANGUS A. MACDONALD. JOHN L. JOHNSON.

